HIV acquisition in men through penile exposure is one of the least studied aspects of HIV transmission. Medical Male Circumcision (MMC) has been shown in clinical trials to reduce the risk of HIV infection in heterosexual men. It is not fully known how MMC works to decrease male heterosexual transmission. One possible explanation is that the urethral and skin mucosal barriers of the penis change after MMC to strengthen its defenses against HIV. Another possibility is that the foreskin itself is particularly vulnerable to HIV, having weaker defenses allow the virus to more easily reach susceptible immune target cells. This might especially be true when an uncircumcised man has a sexually transmitted infection (STI) would generate a local immune response and recruitment of susceptible HIV target cells. These possibilities form the basis of this study which will determine how the penile mucosal barriers change after MMC. We will recruit several cohorts of sexually active males between 18-35 years old receiving MMC in Chicago and Cape Town and from an STI clinic in Cape Town. To address the potential role of STIs, we will also enroll males in Cape Town who are asymptomatically positive for Chlamydia Trachimonas (CT) or Human Papilloma Virus (HPV). To monitor a changing local environment, we will follow these participants for 2-6 months after CT treatment or MMC. We will measure changes in penile skin integrity using in vivo monitoring of trans epithelial water loss (TEWL) and collect foreskins to use in laboratory-based investigations aimed at identifying factors that may lead to increased HIV susceptibility. We will also characterize the penile skin and urethral microbiome and characterize inflammation levels in the urethra. We will explore possible mechanisms for how MMC works to decrease HIV acquisition in men through three specific aims. Aim 1 will determine how circumcision and asymptomatic STI (CT and HPV) influence the urethral immune environment and microbiome. In Aim 2, we will compare differences in the coronal sulcus (CS) microbiome and TEWL pre and post MMC, and assess the potential impact of asymptomatic CT and HPV infections on these measurements. These findings will be linked to microbiome and immune changes in the urethra. Finally, in Aim 3, we will compare target cells, barrier function, structural barrier protein expression, and virus interactions between inner and outer foreskin and determine if infection with an asymptomatic STI can alter the local mucosal environment. We therefore hypothesize that the inner foreskin: 1) has greater permeability, 2) has reduced expression of skin integrity proteins, 3) contains more HIV-1 immune cells and 4) allows for greater HIV attachment and penetration, than the outer foreskin. These interactions may be influences by the presence of asymptomatic CT and HPV. Collectively, our results will provide fundamental knowledge to inform alternative HIV prevention strategies.